1. Technical Field of the Invention
The present invention relates generally to a spark plug which may be employed in automotive engines, and more particularly to an improved structure of a spark plug with a noble metal chip welded to a ground electrode for providing higher durability and ignitability of a gaseous fuel and a fabrication method therefor.
2. Background Art
Japanese Patent First Publication No. 52-36237 (corresponding to U.S. Pat. No. 4,109,633, issued on Aug. 29, 1978 to Mitsudo et al.) teaches a spark plug which consists of a center electrode and a ground electrode which project from electrode supports. The center and ground electrodes are thinner than the electrode support for improving the ignitability of an air-fuel mixture. This is based on the facts that the thinning of the center and ground electrodes results in an decrease in thermal capacity thereof, thereby reducing the effect of extinguishing a flame kernel and that the projection of the ground and center electrodes from the electrode support results in an increased space between the center and ground electrodes, thereby facilitating the growth of a flame kernel produced within a spark gap.
In order to ensure the wear resistance, the center and ground electrodes are formed by noble metal members which are made of Pt, Pd, Au, or alloy thereof and joined to the electrode supports. The publication teaches that such joining may be achieved by welding, pressing, or staking after pressing, but is silent about the details of the shape and structure of a weld of each of the electrodes to one of the electrode support.
In modern engines, a combustible atmosphere is elevated in temperature for increasing an output and reducing a fuel consumption and exhaust emissions. In this type of engine, a park plug is subjected to an intense heat, so that the temperature of center and ground electrodes is increased greatly. The electrodes, therefore, undergo a thermal stress and oxidation, which may cause noble metal chips to be removed from the center and ground electrodes. Particularly, such a problem is exacerbated in the ground electrode because it is closer to a plug housing or metal shell than the center electrode, so that the degree of heat dissipation from the ground electrode is lower, and it has a wider area exposed inside a combustion chamber, so that the temperature thereof is elevated higher than that of the center electrode.
In order to enhance the reliability of welding the noble metal chip to each of the ground and center electrodes, Japanese Patent First Publication No. 9-106880 (corresponding to U.S. Pat. No. 5,811,915, issued on Sep. 22, 1998 to Abe et al., assigned to the same assignee as that of this application) and Japanese Patent First Publication No. 11-354251 teach an improved welding method. The former discloses pressing the noble metal chip against each electrode to have a portion of the electrode surrounding the noble metal chip stand up and radiating a laser beam to the protuberant portion to join the noble metal to the electrode. The latter discloses placing the noble metal chip made of an Ir alloy on each electrode and radiating a laser beam from outside the noble metal chip.
The inventors of this application made several researches and found that the above two welding methods encounter the drawback in that when thin noble metal chips having, for example, a sectional area of a fraction of a square millimeter are used for increasing the ignitability of a gaseous fuel, it is difficult to ensure a desired mechanical strength of the weld of the noble metal chip, especially to the ground electrode because the degree of heat dissipation from the ground electrode is lower than the center electrode, so that the temperature thereof is elevated higher than that of the center electrode.